Why does Demul suffer from such input lag with vsync on, compared to other emulators?

I recently migrated all of my Naomi and Atomiswave games to use Demul, since Demul is so mature now. However, after redoing all of my setup, I started testing games and noticed very strong input lag with vsync on. I have many emulators installed on my cab and this is easily the worst input lag I've seen. The only other one that comes close is one of the SNES cores in RA unless it has been improved recently. I know the alternative is to play with vsync off but that is a deal breaker for me.

Is the input lag likely to be affected by any of the tasks or features currently under development for Demul or is this just how Demul will work for the foreseeable future?

Thanks in advance and please realize I'm not criticizing. Demul is fantastic and I just wish it were more playable with vsync on.

are you familiar with arcade hardware of that era ?
there is looong chain from controls to changes on screen: JVS IO board's CPU read actual controls, store them, then JVS Host MCU via (not fast) serial interface receive controls state from IO board, then MCU send them to CPU RAM via maple interface (serial, so takes some time too), then game code finally got controls status.
then game parse these controls and proceed game logic (which take some time), create display render list, and send it to GPU for render (including 2D-looking games too, actually all games rendered in 3D).
display list transfer and rendering takes 2 or 3 frames, at least, until rendered bitmap will be finally set in display registers and shown onscreen, at next frame.
so, it is nowhere near older low-latency 2D gaming hardware.

yes, I may imagine Xinput/Dinput and Direct3D adds a bit more of delays, but I highly doubt it makes significant difference in addition to how laggy these games and hardware by design.

Thanks for your reply. I'm not trying to be trash demul and I'm definitely not too familiar enough with the hardware to have a feel for how laggy they were originally. It's just that in other emulators, the latency feels significantly lower.

which emulators ? many of them doesn't cared to emulate things how they really was and is, "cut some corners" and it resulted a bit less delay between user action and game reaction.
in the case NullDC or older builds of Reicast - there is primitive and incorrect JVS MCU HLE simulation, which I think reduce delay by 1 frame.
but, about week or so ago Reicast's JVS HLE was significantly improved, now it closer to real thing and also have added frame lag in most cases, so now its close to implementation in Demul (but still wrong in some parts, and I don't think they handle MCU and Maple interface delays right).
there is also video part, other emulators usually show rendered picture onscreen right after render was done, while in real Naomi/Dreamcast and Demul (but not if used DX11Old) its just stored in video memory, and will be showed only if/then 2d display part will be reprogrammed to display it. so, more delay here too. I think in total it might gave 2-3 frames less lag than real Naomi was.

in general, I'm working in touch with wide number of arcade owners/collectors/technicians/etc, also number of (mostly Russian) Dreamcast owners, but I haven't heard from them complains about input lag in this emulator, so I'm assume it is about the same as was on real hardware.
also the goal of project is preservation, of gaming experience on some hardware, as it is.

Also the goal of project is preservation, of gaming experience on some hardware, as it is.

I can certainly understand that. Somebody has to carry the torch of preserving the platform and there will always be other options for casually playing the games. I'll keep an eye on future development. Thanks for taking the time to explain.

I resorted to switching back to gpuDX11old for now since it does feel more responsive and allows for prescaling the video which makes the image sharper. I am also forcing vsync with the nVidia Control Panel but I'm not sure that part actually makes a difference.